This invention relates to a tire testing device, and in particular to a device for testing the tires of aircraft.
1. BACKGROUND TO THE INVENTION
Aircraft tires are required to contain a maximum of 5% oxygen. The reason for this is that if tires become heated to more than approximately 200xc2x0 C. (which may occur if a set of brakes is binding or dragging), the chlorobutyl in the tire material begins to break down and to produce isoprene, and isoprene and oxygen together form an auto-ignitable mixture. The auto-ignition of aircraft tires might have been the cause of an unknown number of previously unexplained losses of aircraft.
The instance of such auto-ignition is likely to increase because it is becoming increasingly common to fit aircraft with carbon brakes. Carbon brakes are advantageous since they are lighter, more efficient and longer-lasting than conventional brakes. However, whilst conventional brakes melt at around 400xc2x0 C., carbon brakes are effective up to around 1100xc2x0 C., so that the temperature which can be generated in a binding brake, and therefore which can be transmitted to an aircraft tire, is correspondingly increased.
Because of the known problems with auto-ignition, there is a mandated limit of 5% oxygen in aircraft tires; the presence of such low concentrations of oxygen prevents auto-ignition in the presence of isoprene.
Aircraft owners and users seek to meet this limit by filling the tires with nitrogen. Aircraft tires may require a pressure of around thirty atmospheres, for example, and so the air which is present in the tire before inflation becomes diluted by around thirty times. The air which was present in the tire before inflation will typically be atmospheric air containing around 21% oxygen; diluting this thirty times with pure nitrogen will result in an oxygen content within the tire of 0.7%, well within the mandated limit.
However, the nitrogen which is used to inflate (or reflate) the tire will seldom if ever be pure, and in certain cases might contain several percent oxygen. It is necessary that the nitrogen supply contain less than 4.3% oxygen (for a thirty atmosphere pressure tire) so that the 5% level can be met.
In many cases, the owners of aircraft will use liquid nitrogen to inflate and reflate the tires of their aircraft, and this supply can be close to 100% pure in practice. However, liquid nitrogen is expensive and other less diligent owners and users instead utilise pressurised nitrogen gas. Often the pressurised nitrogen gas is purchased primarily by price, and the quality (i.e. the percentage of oxygen present in the gas) is not certified and may not be known.
Also, at some airports the ground staff are not qualified or are not trained to appreciate the significance of the oxygen content of the tires, and it has been known that tires be filled from an oxygen supply when the nitrogen supply was not available!
In addition to the oxygen content of the gas within the aircraft tire being critical, the pressure of the gas within the tire is also important. Thus, a tire which is under-inflated does not offer the same level of grip as it is intended to provide, and under-inflated tires are believed to be particularly suspectible to aquaplaning or hydroplaning when there is standing water on the runway (where a layer of water becomes trapped between the tire and the surface of the runway, reducing the level of grip therebetween). There have been a number of aircraft accidents in which the aircraft has skidded off the runway, and aquaplaning is believed to be a likely cause of the aircraft""s failure to stop.
2. DESCRIPTION OF THE PRIOR ART
Pressure testing devices for the tires of aircraft are well known. One form comprises a mechanical gauge similar to that first invented over one hundred years ago. More modern devices use an electromechanical sensor.
However, with such devices it is only feasible to test the pressure of the tires when they are at a known reference temperature, and this typically means that the tire must be at or close to the ambient temperature, otherwise the hot gas within the tire will be at a greater pressure than the corresponding gas when cold, and the temperature-induced variation will render the pressure reading unreliable.
In the United States, for example, the Federal Aviation Authority (F.A.A.) has expressed the wish that tire pressures be tested every day, but the airlines have indicated that this cannot be achieved in practice because aircraft are often in continual operation for up to three weeks at a time, and the aircraft is not on the ground within this period for long enough for the tires to cool sufficiently for reliable testing to take place.
Oxygen testing devices are also available, by which the oxygen content of an aircraft tire can be tested. However, the use of these devices is not universal because of the time taken to undertake the testing. Thus, it has been estimated that to test the pressure and oxygen content of every tire on a large aircraft can take up to two hours, and this is longer than the desired turn-around time for most aircraft (regardless of the time necessary for the tires to cool sufficiently for a reliable pressure test to be carried out).
It is an aim of the present invention to provide a tire testing device which can be used when the aircraft tire is hot or cold.
It is another aim of the present invention to provide a tire testing device which can test the pressure and oxygen content of the tire in a single operation, i.e. only a single application of the valve head upon the tire valve needs to be undertaken.
According to the invention therefore, there is provided a tire testing device including a pressure sensor, characterised in that the device also includes a temperature sensor.
Preferably, the device has means to store a record of the volume of the tire, and means to calculate an effective pressure at a reference temperature. In the preferred embodiments the effective pressure at a reference temperature can be calculated by the device, but in other (less preferred) embodiments the pressure calculation can be carried out separately, e.g. by a computer or other device to which the measured pressure and temperature are downloaded.
It is known that the relationship between the pressure, temperature and volume of a given quantity of gas are related to each other (by Boyle""s Law), and knowledge of the volume of gas within the tire can enable a pressure reading at any particular temperature to be converted to a pressure reading at another (reference) temperature. Accordingly, if it is determined that the pressure of the tires should be measured at 0xc2x0 C., and the actual pressure is measured at 50xc2x0 C., then the equivalent or effective pressure at 0xc2x0 C. can readily be calculated.
The ability of the device to measure the tires at any given temperature, and in the preferred embodiments to calculate the pressure at a reference temperature, enables the device better to check any leakage of gas which is occuring from a tire. Thus, with conventional pressure test apparatus it would still not be possible to obtain any really useful data even if an aircraft""s tires were allowed to cool and be measured every day, if the same aircraft is present in a hot atmosphere such as Arizona on one day, and a cold climate such as Alaska on the following day, since the ambient temperature, and thus the temperature of the xe2x80x9ccoldxe2x80x9d tire, might differ by 40xc2x0 C. between the two locations.
Preferably, the device comprises a base unit and a valve head, the valve head being connected to the base unit by a flexible tube, the valve head being adapted to connect to the tire valve and to allow the escape of a small amount of gas therefrom. Preferably also the temperature sensor is located within the valve head, so that the temperature of the gas is measured as close to the tire as possible, and the likelihood of miscalculations occuring, because for example the gas has cooled on leaving the tire, are much reduced.
Desirably, the temperature sensor is a thermocouple. Desirably also, the pressure sensor is an absolute pressure sensor, i.e. a pressure sensor which can compare a pressure to be measured against vacuum. Usefully, the pressure sensor is a pressure transducer configured as a strain gauge, such as that produced by Kistler of Switzerland under model number MER 180.A.20. The xe2x80x9c20xe2x80x9d element indicates that the sensor can operate over a range of 20 bar which is believed to be sufficient for the majority of applications, but other sensors can be used, providing a greater (or lesser) range, if desired.
Desirably, the device also includes an oxygen sensor. Accordingly, the device can measure the pressure of the gas in the tire and can also measure the oxygen content at substantially the same timexe2x80x94in particular requiring the operator to extract only a single xe2x80x9cshotxe2x80x9d of gas from the tire for both the pressure and oxygen tests. In this way, if the pressure of all of the tires is measured every day, a record of the oxygen content can also be taken every day, and the reflation of the tire with poor quality nitrogen (i.e. containing a large percentage of oxygen), or even with oxygen instead of nitrogen, can quickly be ascertained.
Preferably, the oxygen sensor is located in the base unit. Thus, it is not necessary that the oxygen content be measured close to the valve since this content will not vary between the valve and the base unit. Preferably also, the base unit contains a chamber into which the tire gas can be passed, the oxygen sensor being located within the chamber. Desirably, the device includes means to purge the chamber, so that the gas from a previous tire measurement does not contaminate the test for the oxygen content of the next tire.
Usefully, the base unit is portable, and is ideally designed to be hand-held so that it may be carried around the aircraft, and specifically carried to each tire of the aircraft which is being tested. Preferably, the base unit includes a clip so that it may be suspended from a belt worn by the operator.
Accordingly, the device will be portable and the base unit will preferably contain at least one battery to provide electrical power. The battery is preferably rechargeable, but the device also ideally contains a non-rechargeable battery bay, so that non-rechargeable batteries can be purchased and used in the event that the rechargeable battery has not been sufficiently recharged prior to use.
Desirably, the base unit includes interface means by which the data stored therein can be downloaded to a computer such as a mainframe computer. The device can have a volatile memory in which can be stored the data corresponding to the oxygen content, the pressure measurement and the calculated effective pressure, of the tires of each aircraft which the operator has tested, and a permanent record of those measurements can be kept within the computer or outputted therefrom in permanent form.
Usefully, the device takes periodic measurements of the ambient atmosheric pressure and temperature, and can utilise those measurements in converting the measured pressure of the tire gas to the effective (reference) pressure. Thus, it is necessary also to account for the atmospheric pressure when calculating the effective pressure, and the device can cater for changes in the atmospheric pressure when making the effective pressure calculations. Accordingly, the pressure can be calculated at a reference temperature and also at a reference atmospheric pressure, so that the pressure within the tires of the same aircraft can be reliably measured at both sea level (e.g. Miami) and altitude (e.g. Denver), without the relative change in atmospheric pressure between those locations affecting the reliability of the effective pressure reading.
Preferably, the device has a xe2x80x9cstand-byxe2x80x9d mode in which the display may be switched off during periods of inactivity. However, the ambient pressure and temperature readings can continue to be taken periodically during these stand-by periods. The device can return to full operational mode when the operator activates a switch, though preferably this is automatic, and for example occurs when a significant rise in pressure or temperature is detected, indicating that a measurement is to take place.
Since the device will need to include a record of the volume of the tire, it has a permanent memory containing a database of the volume of the tires of the particular user""s aircraft. Thus, the volume of gas within all available aircraft tires can be calculated or referred to, and the permanent memory of the device can be used to store the volume of the tires of each of the user""s aircraft. In addition, the device will preferably include a display means by which the operator will be directed sequentially to each tire of the aircraft, and the device will automatically calculate the effective pressure by referring to the volume of each particular tire stored within its permanent memory.
The device will preferably have a control button which permits the operator to select the aircraft which is to be tested; the selection may be made by aircraft type, but preferably is made by reference to the aircraft""s unique alphanumeric code, so that a record of the pressure and oxygen content of each tire of each aircraft can be maintained.
Desirably, the device has a display screen. Desirably also the control means of the device can cause a representation of the aircraft to appear on the display screen, the control means also identifying each tire of the aircraft upon the display screen in the sequential order in which the tires should be tested.